1. Field of the Invention
The present invention relates to a display apparatus that displays an image by irradiating light from a light source to display means, a quantity-of-light adjusting method for a display apparatus and electronic equipment.
2. Description of the Related Art
Liquid crystal displays including a liquid crystal television may use an LED (or light emitting diode) device as a back light thereof for one advantage that the range of the luminance control is wider than that of a CCFL (Cold Cathode Fluorescent Lamp) (refer to JP-A-2005-310997 (Patent Document 1), for example).
The CCFL may also control the luminance and mainly includes two methods of voltage light control and current light control. The former is a method that feeds back the voltage to be applied to a transformer and adjusts the voltage for light control and has a light control range of 50 to 100% generally.
The latter is a method that feeds back output current and adjusts the voltage to be applied to a transformer for light control and has a light control range of approximately 50% to 100%, which is equal to that of the voltage light control. PWM (Pulse Width Modulation) light control is an alternative method. This method has an extended range of light control of approximately 10% to 100%.
Therefore, it is difficult to obtain a range of 10% or smaller even by performing PWM light control, and in order to obtain a light control range of 10% or smaller, it is said that a back light employing an LED device is superior.
A recent high-quality-demanded display apparatus such as a television may be driven by a control system to detect a color temperature at all times and receives the feedback for keeping a constant chromaticity since a constant color temperature is important even with various luminance levels.
Accordingly, control over luminance by using an LED back light is important. Here, the method for luminance control over an LED back light may be:
[1] PWM (Pulse Width Modulation) method that adjusts luminance in time;
[2] A method that adjusts current (current peak value) to be fed to an LED; or
[3] A method that uses [1] and [2]
The PWM method is a pulse modulation method that changes and modulates the width of pulses at a constant amplitude during a predetermined period according to the pulse width modulation and modulation signals. As the amplitude of signal waves increases, the width of pulses increases. As the amplitude decreases, the width of pulses decreases.
An LED back light is assumed here as an example that includes an LED array of a red LED, a green LED and a blue LED. Notably, there are no reasons for configuring LEDs for three colors as above, LEDs for other colors excluding the three colors may be mixed.
The three methods for controlling the luminance of the LED back light will be described more specifically.
[1] Case where PWM is Adjusted for Luminance Control
In PWM on the RGB LEDs, the pulse width is adjusted for each of the RGB LEDs to obtain an arbitrary white balance. If PWM is defined for a higher RGB lighting rate (such as 50% or higher), the light control only with PWM can keep a constant current value and keep the linearity in the relationship between PWM and the luminance. Defining PWM for a lower RGB lighting rate (such as 10% or lower) produces narrower current waveforms, which are susceptible to the rising and falling characteristics. From the viewpoint of designing a circuit for driving an LED, it is important to design a sophisticated LED driver that can stably output a current value and PWM even with a lighting rate of 10% or lower.
[2] Case where Current (or Current Peak Value) to be Fed to LED is Adjusted for Luminance Control
The method that performs luminance control by adjusting a current peak value is required to adjust the current wave height value of lower current. Therefore, also in this case, sophisticated design is important for the driver circuit that lights LEDS.
[3] Case where Both PWM and Current Peak Value are Adjusted for Luminance Control
The luminance control by adjusting both PWM and current peak value can extend the light control range more than those of [1] and [2]. However, it may disadvantageously complicate the algorithm for controlling the luminance to decrease or increase with the chromaticity kept constant. Preferably for simple control, one of PWM and the current peak value is used as a variable for luminance control, and the other is used for keeping the chromaticity.